Disk wind in young binaries and its impact on the infrared excesses of young stars

We consider the effect of binarity of young stars on the spectral energy distribution of the IR radiation from circumstellar dust. The formation of a common dust envelope in a binary system with a low-mass secondary component is strongly affected by the disk wind from the secondary. The small velocities in peripheral areas of the wind are such that it can be partially or entirely captured by the primary, even when the distance between the components is up to several astronomical units. As a result, an envelope with a rather complex spatial and kinematic structure is formed. Its mass is many orders of magnitude smaller than that of the accretion disk around the binary. However, the thermal radiation emitted by dust particles of the envelope can be comparable to the total radiation of the accretion disk. This result is discussed in the context of the deficit at near-IR wavelengths (2–10µm) in current models for accretion disks around young stars.     

Three-dimensional hydrodynamical modeling of the two-component wind and accretion disk in the close binary β Lyrae

This paper continues a series of studies on three-dimensional hydrodynamical modeling of mass transfer in the binary system β Lyr. The model takes into account the stellar wind from the donor star, which outflows at a rate of , as demonstrated by radio observations. This stellar wind should appreciably influence the formation of the envelope in the binary. Computations have shown that the interaction of the matter flow from the Lagrangian point L₁ and the accretor wind leads to the formation of an optically and geometrically thick gaseous envelope around the accretor. The matter flow meets the accretor wind, spreads out, accumulates over the outer edge of the wind, and forms a geometrically thick envelope (disk). The wind flows freely at the center of the disk, over the accretor poles. Jet-like structures arise beyond the wind-propagation region, above the thick accretion disk. The matter flowing from the outer edge of the disk interacts with the donor wind, leading to the formation of a standing shock between L₁ and the outer edge of the disk, in the direction corresponding to orbital phase 0.25. This shock is able to explain the origin of the X-ray radiation from the binary β Lyr.     

Reverse rotation of the accretion disk in RW Aur A: Observations and a physical model

Speckle interferometry of the young binary system RW Aur was performed with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences using filters with central wavelengths of 550 nm and 800 nm and pass-band halfwidths of 20 nm and 100 nm, respectively. The angular separation of the binary components was 1.448?? ± 0.005?? and the position angle of the system was 255.9° ±0.3° at the observation epoch (JD 2 454 255.9). We find using published data that these values have been changing with mean rates of +0.002??/yr and +0.02°/yr, respectively, over the past 70 years. This implies that the direction of the orbital motion of the binary system is opposite to the direction of the disk rotation in RW Aur A. We propose a physical model to explain the formation of circumstellar accretion disks rotating in the reverse direction relative to young binary stars surrounded by protoplanetary disks. Our model can explain the characteristic features of the matter flow in RWAur A: the high accretion rate, small size of the disk around the massive component, and reverse direction of rotation.     

The mechanism of circumbinary envelope formation in close binaries

We consider the structure and formation of the circumbinary envelopes in semi-detached binary systems. Three-dimensional numerical simulations of the gas dynamics are used to study the flow pattern in a binary system after it has reached the steady-state accretion regime. The outer parts of the circumbinary envelope are replenished by periodic ejections from the accretion disk and circum-disk halo through the vicinity of the Lagrange point L₃. In this mechanism, the shape and position of a substantial part of the disk is specified by a precessional density wave. On timescales comparable to the orbital period, the precessional wave (and hence an appreciable fraction of the disk) will be virtually stationary in the observer’s frame, whereas the positions of other elements of the flow will vary due to the orbital rotation. The periodic variations of the positions of the disk and the bow shock formed when the inner parts of the circumbinary envelope flow around the disk result in variations in both the rate of angular-momentum transfer to the disk and the flow structure near L₃. All these factors lead to a periodic increase of the matter flow into the outer layers of the circumbinary envelope through the vicinity of L₃. The total duration of the ejection is approximately half the orbital period.     

The structure of the common envelope in a close binary system

As a result of the interaction between an elliptical accretion disk and gas flowing into it from the circumbinary envelope in a close binary in the course of its orbital motion, the matter of the disk and the circum-disk halo is periodically ejected from the vicinity of the Lagrange point L₃, and a common envelope is formed in the system. Three-dimensional numerical gas-dynamical modeling is used to study the structure and dynamics of the envelope and determine its basic parameters. The evolution of the envelope is followed on timescales of the order of several orbital periods. The matter flow ejected through the vicinity of L₃ displays a spiral shape. The maximum size of the forming spiral structure is restricted by the self-intersection point, and is of the order of four to five times the component separation. We consider the dynamics of the regions directly adjacent to the spiral structure: an inner, rarified and outer, fragmented region, which further makes a transition to an expanding diffuse ring.     

Observational manifestations of the common envelope in a close binary

Three-dimensional (3D) numerical models of close binaries are used to study the structure and dynamics of common envelopes formed due to periodic ejections of matter from the accretion disk through the vicinity of the Lagrange point L₃. The results are used to estimate the physical parameters of the envelope, including its 3D matter distribution, and the matter-flow configuration and dynamics. Possible observational manifestations of such envelopes are estimated. We present the envelope’s radialvelocity distributions at various phases and times, as well as model light curves taking into account extinction in the envelope. The envelope becomes optically thick for systems with high mass-exchange rates, ? > 10^?8 M _⊙/year, and has a significant influence on the binary’s observed features. The uneven phase distributions of the matter and density variations due to periodic injections of matter into the envelope are important for interpretations of observations of close binary stars.     

Envelope structure in T Tauri binary stars with subsonic orbital motion of one component

Results of numerical modeling of the gas dynamics ofmaterial in the envelopes of T Tauri binary stars with a small component mass ratios (q = 0.08) are reported. In such systems, the less massive component is moving at a supersonic velocity, and the more massive component can move with either a subsonic or supersonic velocity. The modeling results show that the morphology of the flow changes substantially in the transition from supersonic to subsonic motion of the massive component. In particular, one of the two bow shocks vanishes, flows ofmaterial in the system are redistributed, and the characteristics of the accretion disks change. In addition, the effect of the change in the accretion mode on the evolution of the binary system and the possibility of recovering some parameters of the system from observational manifestations of shocks in the circumstellar envelope are considered.     

Hydrodynamical modeling of mass transfer in the close binary system <Emphasis Type="Italic">β</Emphasis> Lyr

We present results of two-dimensional hydrodynamical simulations of mass transfer in the close binary system β Lyr for various radii of the accreting star and coefficients describing the interaction of the gaseous flow and the main component (primary). We take the stellar wind of the donor star into account and consider various assumptions about the radiative cooling of the gaseous flow. Our calculations show that the initial radius of the flow corresponding to our adopted mass-transfer rate through the inner Lagrange point (L₁) of (1–4) × 10^?5M_⊙/yr is large: 0.22–0.29 (in units of the orbital separation). In all the models, the secondary loses mass through both the inner and outer (L₁ and L₂) Lagrange points, which makes the mass transfer in the system nonconservative. Calculations for various values of the primary radius show a strong dependence on the coefficient fv that models the flow-primary interaction. When the radius of the primary is 0.5, there is a strong interaction between the gas flow from L1 and the flow reflected from the primary surface. For other values of the primary radius (0.1 and 0.2), the flow does not interact directly with the primary. The flow passes close to the primary and forms an accretion disk whose size is comparable to that of the Roche lobe and a dense circum-binary envelope surrounding both the disk and the binary components. The density in the disk varies from 10¹² to 10¹⁴ cm^?3, and is 10¹⁰–10¹² cm^?3 in the circum-binary envelope. The temperature in the accretion disk ranges from 30000 to 120000 K, while that in the circum-binary envelope is 4000–18000 K. When radiative cooling is taken into account explicitly, the calculations reveal the presence of a spiral shock in the accretion disk. The stellar wind blowing from the secondary strongly interacts with the accretion disk, circum-binary envelope, and flow from L₂. When radiative cooling is taken into account explicitly, this wind disrupts the accretion disk.     

Formation of a radiative wind and accretion disk in microquasars. Generation of flare activity in the disk due to an increase in the supply of material to the Lagrange point L<Subscript>1</Subscript>. The system LMC X-3

Three-dimensional numerical hydrodynamical modeling of a radiative wind and accretion disk in a close binary system with a compact object is carried out, using the massive X-ray binary LMC X-3 as an example. This system contains a precessing disk, and may have relativistic jets. These computations show that an accretion disk with a radius of about 0.20 (in units of the component separation) forms from the radiative wind from the donor when the action of the wind on the central source is taken into account, when the accretion rate is equal to the observed value (about 3.0 × 10^?8 M _⊙/year, which corresponds to the case when the donor overflows its Roche lobe by nearly 1%). It is assumed that the speed of the donor wind at infinity is about 2200 km/s. The disk that forms is geometrically thick and nearly cylindrical in shape, with a low-density tunnel at its center extending from the accretor through the disk along the rotational axis. We have also modeled a flare in the disk due to short-term variations in the supply of material through the Lagrange point L₁, whose brightnesses and durations are able to explain flares in cataclysmic variables and X-ray binaries. The accretion disk is not formed when the donor underfills its Roche lobe by 0.5%, which corresponds to an accretion rate onto the compact object of 2.0 × 10^?9 M _⊙/year. In place of a disk, an accretion envelope with a radius of about 0.03 forms, within which gas moves along very steep spiral trajectories before falling onto the compact object. As in the accretion-disk case, a tunnel forms along the rotational axis of the accretion envelope; a shock forms behind the accretor, where flares occur in a compact region a small distance from the accretor at a rate of about six flares per orbital period, with amplitudes of about 10 ^m or more. The flare durations are two to four minutes, and the energies of individual particles at the flare maximum are about 100–150 keV. These flares appear to be analogous to the flares observed in gamma-ray and X-ray burst sources. We accordingly propose a model in which these phenomena are associated with massive, close X-ray binary systems with component-mass ratios exceeding unity, in which the donor does not fill its Roche lobe. Although no accretion disk forms around the compact object, an accretion region develops near the accretor, where the gamma-ray and X-ray flares occur.     

Structure of the circumbinary envelope around a young binary system

The structure of a circumstellar envelope around a young binary T Tauri star is considered. The supersonic orbital motion of the system components in the envelope gas leads to the formation of bow shocks around the star. Two- and three-dimensional numerical modeling indicates an important role of these shocks in the formation of the structure of the circumbinary envelope. In particular, for systems with circular orbits, the size of the central region of the envelope that is not filled with matter (the “gap”) is essentially determined by the parameters of the bow shocks. These modeling results are supported by comparisons of the obtained estimates for the gap parameters with observations.     

Two-dimensional hydrodynamical modeling of mass transfer in semidetached binaries with asynchronously rotating components

We have modeled the mass transfer in the three semidetached binaries U Cep, RZ Sct, and V373 Cas taking into account radiative cooling both implicitly and explicitly. The systems have asynchronously rotating components and high mass-transfer rates of the order of 10^?6M_⊙/yr; they are undergoing various stages of their evolution. An accreting star rotates asynchronously if added angular momentum is redistributed over the entire star over a time that exceeds the synchronization time. Calculations have indicated that, in the model considered, mass transfer through the point L₁ is unable to desynchronize the donor star. The formation of an accretion disk and outer envelope depends on the component-mass ratio of the binary. If this ratio is of the order of unity, the flow makes a direct impact with the atmosphere of the accreting star, resulting in the formation of a small accretion disk and a relatively dense outer envelope. This is true of the disks in U Cep and V373 Cas. When the component-mass ratio substantially exceeds unity (the case in RZ Sct), the flow forms a large, dense accretion disk and less dense outer envelope. Taking into account radiative cooling both implicitly and explicitly, we show that a series of shocks forms in the envelopes of these systems.     

The Hydrogen and Helium Lines of the Symbiotic Binary Z And during Its Brightening at the End of 2002

High resolution observations in the region of the Hα, HeII λ 4686, and Hγ lines in the spectrum of the symbiotic binary Z And were performed during a small-amplitude flare at the end of 2002. The profiles of the hydrogen lines were double-peaked, and suggest that the lines may be emitted mainly by an optically thin accretion disk. Since the Hα line is strongly contaminated by emission from the envelope, the Hγ line is used to investigate the properties of the accretion disk. The Hα line has broad wings, believed to be determined mostly by radiation damping, although the high-velocity stellar wind from the compact object in the system may also contribute. The Hγ line has a broad emission component, assumed to be emitted mainly from the inner part of the accretion disk. The HeIIλ 4686 line also has a broad emission component, but is believed to arise in a region of high-velocity stellar wind. The outer radius of the accretion disk can be calculated from the shift between the peaks. Assuming that the orbital inclination can range from 47° to 76°, we estimate the outer radius to be 20–50 R_⨀. The behavior of the observed lines can be interpreted in the model proposed for the line spectrum during the first large 2000–2002 flare of this binary.     

The accretion activity of the Ae Herbig star CQ Tau

Results of spectral observations of the isolated Ae Herbig star CQ Tau obtained in 1995–1998 in the H_α line and near the sodium resonance doublet are presented, together with simultaneous photometric monitoring of the star. CQ Tau is a member of the family of young UX Ori stars with nonperiodic Algol-like brightness decreases. The star is surrounded by an accretion disk, in which its emission-line spectrum and part of its absorption-line spectrum are formed. The strong variability of the H_α, D NaI, and HeI 5876 Å lines testifies that the gaseous disk is appreciably inhomogeneous in both the radial and azimuthal directions. This inhomogeneity probably results from uneven feeding by the circumstellar material from the peripheral regions of the circumstellar disk. Over the four years of observations, we observed the star in deep minima three times (ΔV～2). On these nights, an increase in the H_α equivalent width followed the decrease in radiation flux. In the two deepest minima, the normally two-component line profile had only a single component with a nearly symmetrical profile. This behavior of the H_α line is in good agreement with the results of numerical modeling of Algol-like minima and can be used to estimate the parameters of the dust clouds eclipsing the star and inner accretion disk. These estimates suggest that the circumstellar dust clouds can approach very close to the star and be sublimated there.     

Non-steady-state accretion disks in X-ray novae: Outburst models for Nova Monocerotis 1975 and Nova Muscae 1991

We have fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova Muscae GS 1991=1124-683) using a non-steady-state accretion-disk model. The model is based on a new solution for a diffusion-type equation for non-steady-state accretion and describes the evolution of a viscous α disk in a binary system after the peak of the outburst, when the matter in the disk is totally ionized. The accretion rate in the disk decreases according to a power law. We derive formulas for the accretion rate and effective temperature of the disk. The model has three free input parameters: the mass of the central object M, the turbulence parameter α, and the normalization parameter δt. The results of the modeling are compared with the observed X-ray and optical B and V light curves. The estimates for the turbulence parameter α are similar: 0.2–0.4 for A 0620-00 and 0.45–0.65 for GS 1124-683, suggesting a similar nature for the viscosity in the accretion disks around the compact objects in these sources. We have also derived the distances to these systems as functions of the masses of their compact objects.     

Formation of planets during the evolution of single and binary stars

Current views of the origin and evolution of single and binary stars suggest that the planets can form aroundmain-sequence single and binary stars, degenerate dwarfs, neutron stars, and stellarmass black holes according to several scenarios. Planets can arise during the formation of a star mainly due to excess angular momentum leading to the formation of an accretion-decretion disk of gas and dust around a single star or the components of a binary. It is the evolution of such disks that gives rise to planetary systems. A disk can arise around a star during its evolution due to the accretion of matter from dense interstellar clouds of gas and dust onto the star, the accretion of mass froma companion in a binary system, and the loss of matter during the contraction of a rapidly rotating star, in particular, if the star rotates as a rigid body and the rotation accelerates with its evolution along the main sequence. The fraction of stars with planetary systems is theoretically estimated as 30–40%, which is close to the current observational estimate of ∼34%.     

Three-dimensional hydrodynamical modeling of mass transfer in the close binary system β Lyr

We present a three-dimensional hydrodynamical modeling of mass transfer in the close binary system β Lyr taking radiative cooling into account explicitly. The assumed mass-transfer rate through the first Lagrangian point L₁ is 3.0 × 10^?5 M _⊙/yr. A flow with a radius of 0.14–0.16 (in units of orbital separation) is formed in the vicinity of L₁. This flow forms an accretion disk with a radius close to 23 R _⊙ and a thickness of about 10 R _⊙. The accretion disk is surrounded by an outer envelope that extends beyond the computational domain. A spiral shock forms at the outer boundary of the disk at orbital phase 0.25. Geometrically, the disk is toruslike, while the outer envelope is cylinder-like. In this model, which has low temperatures inside the computational domain, no jetlike structures form in the disk. It is possible that the jetlike structure in β Lyr arises due to the interaction of radiative wind from the accretor with the flow from L₁. In the model considered, a hot region exists over the poles of the accretor at a height of about 0.21. The amount of matter lost by the system is close to 10% of the mass flowing through L₁; i.e., the mass transfer in the system is almost conservative. For a mass-transfer rate of 3.0 × 10^?5 M _⊙/yr, the orbital period varies by 40.4 s/yr. This means that the observed variation of the orbital period of 19 s/yr should correspond to a mass-transfer rate close to 1.0 × 10^?5 M _⊙/yr.     

Interaction of the stream from <Emphasis Type="Italic">L</Emphasis><Subscript>1</Subscript> with the outer edge of the accretion disk in a cataclysmic variable

Vertical oscillations of the gas at the outer edge of the accretion disk in a semi-detached binary due to interaction with the stream of matter from the inner Lagrangian point L ₁ are considered. Mixing of the matter from the stream from L ₁ with matter of the disk halo results in the formation of a system of two diverging shocks and a contact discontinuity, or so-called “hot line”. The passage of matter through the region of the hot line leads to an increase in its vertical velocity and a thickening of the disk at phases 0.7?0.8. Subsequently, the matter moving along the outer edge of the disk also experiences vertical oscillations, forming secondary maxima at phases 0.2?0.4. It is shown that, for systems with component mass ratios of 0.6, these oscillations will be amplified with each passage of the matter through the hotline zone, while the observations will be quenched in systems with component mass ratios ~0.07 and ~7. The most favorable conditions for the flow of matter from the stream through the edge of the disk arise for component mass ratios ~0.62. A theoretical relation between the phases of disk thickenings and the component mass ratio of the system is derived.     

Three-dimensional hydrodynamical modeling of accretion-disk formation in microquasars. Cen X-3

We have carried out three-dimensional hydrodynamical modeling of the formation of an accretion disk around a compact object due to radiative wind of a massive donor in a close binary system. The massive X-ray binary Cen X-3, which has a precessing accretion disk and may possess relativistic jets, is considered as an example. The computations show that, when the action of the central compact object on the formation of the wind is taken into account, the radiative wind forms an accretion disk with a radius of 0.16 (in units of the orbital separation), which accretes at a rate close to 1 × 10^?8 M _⊙/yr. In this model, the disk is spherically symmetrical and geometrically thick, with a tunnel going from the accretor to the upper layers of the disk along the accretor’s rotational axis at the disk center. The number density of the gas in the tunnel is five orders of magnitude lower than in the disk. The wind-disk interaction at the outer boundary of the disk produces a strong shock (wind-disk shock) directed toward the donor. The black-body emission of the disk and tunnel is nonstationary, and resembles the outbursts observed in Cen X-3. An analysis of the location of the region of nonstationary emission suggests that the outbursts occur in the wind-disk shock.     

Gamma-Ray Emission During the Accretion of Matter from a Supernova Envelope onto a Compact Remnant

The aim of this study is to investigate the accretion of matter onto a compact gravitating remnant (neutron star) in the central region of the expanding shell of a Type II supernova. Computations of an explosion with the energetics of a Type II supernova have been performed to derive the structure of matter in the vicinity of the neutron star. The energy of the expanding shell and the parameters of the presupernova correspond to the known values for SN 1987A. This accretion leads to the formation of a layer of fairly dense and hot gas at the surface of the compact remnant, providing the conditions for nucleosynthesis reactions. Thus, one result of the study is to demonstrate the importance of the r and rbc processes, or explosive nucleosynthesis, in the compact envelope of a neutron star. A second result is the production of emission lines from unstable elements formed in the central part of the neutron-star envelope.

     

Turbulence in the Accretion Disks in Semi-Detached Binary Systems with Various Component-Mass Ratios

Results of three-dimensional gas-dynamical numerical simulations of the structure of matter flows in semi-detached binary systems with various component-mass ratios are considered. The main elements of the flows in the models studied are described. The characteristics of density waves inside the accretion disk for various component-mass ratios are considered. The influence of the precessional density wave on the development of instability in the accretion disks and the characteristics of developing turbulence are analyzed. Values of the Shakura–Syunyaev coefficient α for the simulated systems are calculated.